Women in the reproductive age groups currently use both physician and hospital services at almost 1 1/2 times the rate of men in this age group, exclusive of utilization associated with pregnancy (Nathanson 1977). As health service utilization is an issue of national concern, this study will attempt to delineate some of the factors that account for this difference in male and female utilization rates. Health and mortality statistics in the United States have shown that women have higher illness rates but lower death rates than men. Women report more days of illness, more days of disability and more incidence of morbidity than men. However, women live longer and have less severe morbidities than men (Vergrugge 1976). This enigma has not been examined closely nor is it understood. One possible explanation for this sex differential in health service utilization is what Mechanic refers to as illness behavior: the degree to which attentiveness is given to symptoms and how they are evaluated; the degree to which changes in life regimen are made; the claims made on others; and the extent to which help is sought. Also posited in the literature is the notion that women's role obligations are more compatible with the sick role than men's. Their roles may allow more leeway in which to adopt the sick role. This research will examine the differences between men and women in illness behavior, attitudes and beliefs and the degree to which illness behavior accounts for the sex differential in utilization rates. To explain differences among women, an analysis of the number and character of role obligations as a predictor of utilization will be explored. Specifically, the research will assess the effects of illness behavior on utilization and the effect of increasing role obligations on female illness behavior, health and utilization. The 1847 subjects for this research were interviewed in a 1970-71 survey of a sample of the Kaiser-Permanente membership for whom up to seven years of utilization data (both before and after the survey) have been recorded from medical records.